Recently, quantum key distribution (QKD) as a method to realize a cryptographic communication having information-theoretical security against eavesdropping has been actively studied (NPL 1) and its development for practical use has been underway. Especially, for safety improvement, the “decoy method” of detecting eavesdropping attacks by changing from time to time the intensity of faint light pulses used for QKD is employed in most QKD systems for practical use (refer to NPL 2).
Since a “single photon” including only one photon in one pulse (or a “pseudo single photon” obtained by extremely attenuating normal laser light) is used as a communication medium in QKD, a photon detector capable of detecting a single photon is used instead of a light detector as used in a conventional optical communication. An avalanche photodiode (APD) to which a bias exceeding a breakdown voltage is applied or a superconducting element cooled to a few degrees K are generally used as the photon detector.
A variety of methods for QKD have been proposed, and a method of using two or four photon detectors is a common practice (for example, refer to NPL 3). Although it is desirable that characteristics of all photon detectors are as uniform as possible in order to ensure the security of cryptographic keys generated by QKD if a plurality of photon detectors are used, the characteristics of APD elements and superconducting elements generally have large variations and also vary due to fluctuations of environmental temperatures and the deterioration of the elements. Therefore, it is necessary to check the characteristics regularly in operating QKD for a long time and to equalize the characteristics by individually adjusting external parameters such as a bias voltage.
There are quantum efficiency and dark count probability as principal parameters indicating characteristics of photon detectors. The quantum efficiency is a probability of outputting a detection signal correctly when a photon detector receives pulses including a single photon (a detection probability of a photon). The dark count probability is a probability of outputting a detection signal even though there are no photons, and it represents a magnitude of noise. Typically, the quantum efficiency is about 10% and the dark count probability is at the same level as 10−5.